A concentration sensor device is widely used for detecting a concentration of a specific component contained in a variety of liquids. For example, recently, use of alcohol-mixed fuel is promoted, in which a biological origin component such as alcohol is mixed in an oil-origin component such as gasoline or light oil. Properties or controls of an internal combustion engine are changed by a mixture ratio between the oil-origin component and the biological origin component. The mixture ratio may correspond to an alcohol concentration. Therefore, the alcohol concentration of the mixture fuel is required to be accurately detected.
Thus, a concentration of a specific component of liquid is required to be accurately detected in a variety of fields, other than the alcohol-mixed fuel. JP-A-5-507561 discloses a concentration sensor device to detect an alcohol concentration of mixture fuel, for example.
The sensor device disclosed in JP-A-5-507561 includes a casing defining a passage through which fuel flows, and a sensor element arranged in the casing. The sensor element is exposed to mixture fuel flowing through the passage, and detects the alcohol concentration by directly contacting the fuel. However, in JP-A-5-507561, the passage defined by the casing has a complicated labyrinth shape, and the sensor element having a large size is arranged in the complicated passage. Therefore, foreign solid object or air bubble contained in the fuel easily adheres onto a detector of the sensor element. As a result, accuracy for detecting the concentration of the specific component contained in fuel may be lowered, due to the adhesion of the foreign object or the bubble.
JP-A-5-87764 discloses a device to measure an alcohol amount (a ratio of alcohol to gasoline) in mixture liquid containing alcohol and gasoline. The device includes an electrode, a sensor element and an electronic measuring circuit. The electrode defines a part of casing through which the mixture liquid flows. The sensor element is arranged in the casing. The electronic measuring circuit evaluates a capacitance of a capacitor defined by the electrode and the sensor element. The electronic measuring circuit detects a permittivity of the mixture liquid based on the capacitance, and calculates a mixture ratio of alcohol to gasoline based on the detected permittivity.
The electrode and the sensor element are located in the mixture liquid so as to detect the capacitance representing the permittivity of the mixture liquid. Foreign objects contained in the mixture liquid may adhere onto the electrode or the sensor element, thereby the capacitance of the capacitor defined by the electrode and the sensor element may be varied. Because a permittivity of the foreign object contained in the mixture liquid is typically larger than that of the mixture liquid, the detected capacitance may be larger than an expected value. When the capacitance of the capacitor is varied by the foreign object, a relative permittivity calculated based on the capacitance is varied, and the mixture ratio of alcohol to gasoline calculated based on the relative permittivity is varied. Thus, accuracy for detecting the mixture ratio of alcohol to gasoline may be lowered.
Recently, alcohol receives attention as an alternative fuel for gasoline. For example, mixture fuel starts to widespread such as a bio-gasoline containing gasoline and ethanol as a main component, or bio-mixed light oil containing light oil and fatty acid methyl ester as a main component. When the mixture fuel is used, concentrations of gasoline and alcohol are timely detected so as to obtain alcohol amount, such that fuel amount suitable for operation state of an internal combustion engine is injected to a combustion chamber. JP-A-5-87764 or JP-A-2008-268169 discloses a concentration detecting device to detect concentrations of gasoline and alcohol.
For example, the detecting device disclosed in JP-A-5-87764 detects alcohol concentration. Mixture fuel (mixture fluid) containing gasoline and alcohol to be measured is introduced between electrodes located in a casing, and a capacitance (permittivity) of the mixture fuel is measured. Further, a liquid properties sensor disclosed in JP-A-2008-268169 detects a mixture ratio of alcohol to gasoline. Comb-teeth shape electrode is arranged on a semiconductor substrate, and the substrate is immersed in liquid fuel such as gasoline, so as to measure a capacitance (permittivity).
However, the mixture fuel may contain water, for example, other than gasoline and ethanol. Specifically, water may be mixed in ethanol in a purification process. Further, water contained in air may melt in the mixture fuel, when the mixture fuel contacts air. Furthermore, water may be mixed in the mixture fuel by human error, when the mixture fuel is transported. When water is mixed into the mixture fuel, the capacitance (permittivity) of the mixture fuel is varied based on concentrations (ratio) of gasoline, ethanol and water. However, the mixture fuel is defined to contain only gasoline an alcohol, and only the concentrations of gasoline and alcohol are measured. Thus, water may generate measurement error.